In order to obtain optical devices with improved performance, quantum-dot based devices using structures such as InAs/InGaAsP are becoming increasingly attractive. Typically, the substrate is made of InP and the active material is InAs Quantum dots embedded within an InGaAsP barrier. Examples of such devices are laser sources, optical amplifiers, optical sensors (detectors) or the like. The interest in using the quantum-dot based devices lies mainly in their ability to provide low chirp, low noise, low sensitivity to temperature and broad wavelength applications.
Quantum-well and quantum-dot semiconductor structures are well known in the related art. In brief terms, a quantum-well structure is arranged in such a way as to allow for propagation of electrons or holes in two spatial dimensions while limiting such propagation in a third dimension, whereas a quantum-dot structure is arranged in such a way so as to confine the carriers in three dimensions.
One advantage of using a quantum-dot structure for an optical device is that with such structures a relatively low timing-jitter in actively mode-locked lasers may be achieved.
However, quantum-dot based devices suffer from a relatively high sensitivity as regards polarization. This is a drawback because of the following reasons:
As it is known, an optical signal transmitted from an optical source, may be polarized. This means that the direction of oscillation of components of the optical signal is perpendicular to the direction of propagation of the optical signal itself. A well-known example of polarization is one having an electrical component and a magnetic component known as TE mode and TM mode respectively. The transmitted optical signal usually travels through an optical fiber towards an optical receiver. In practice, when a polarized optical signal travels along an optical fiber, it undergoes certain distortion as regards its polarization status. This is because the optical fibers are usually not capable of maintaining the modes of polarization in the original status as they are transmitted from the optical source. As a consequence, the optical signal is received at the receiver end with distorted polarization. Therefore, if the receiver is a polarization sensitive device, it will not be able to correctly process the received optical signal. As already mentioned above, quantum-dot based devices are polarization sensitive and therefore suffer from this drawback.
In order to overcome this problem, certain solutions are known. One of such solutions relates to cascading a bulk-based laser and a quantum-dot based laser. However, the cascading of the two devices results in structural complexities, increased size and cost and it may give rise to coupling losses.
Another solution relates to splitting the optical signal in order to change its polarization. In such cases, the input signal is split in order to separate the signal in two polarizations, TM and TE. The TM part is then rotated by a polarizer and recombined with the TE part before being sent in the SOA. However, this solution is not easy to implement because it would be required to avoid destructive interference between the two arms during the recombination process and furthermore it gives rise to substantial increase in packaging costs.